Thorax (CXR)

Question 1

How are x-rays formed?

Answer 1

• photons of energy interacting with the body, essentially producing a shadow of the structures on some detection device
  
  • photographic film: chemical rxn –> silver halide crystals to silver (black); where more x-rays passed through, image was blacker and vv
  • negative film using slide projection
  • now done on digital detectors and computers
• disadvantages: planar image only, collapses 2D object into 3D; no depth analysis
  
  • only get 3D perspective with another projection e.g. lateral
• advantages: cheap and easy

Question 2

How are chest x-rays taken?

Answer 2

• full inspiration
• PA with x-ray to pt back, and heart closest to the film
• ‘hug’ film to move scapulae out of the way
• erect
  
  • to determine blood flow distribution to the lungs
    
    • less blood flow to top of n. lung (larger vessels tf at bottom)
    • in LHF, +pulm P, vessels at top are enlarged (pulm vascular distribution)
  • to look for pleural fluid, sinks to bases of lungs/pleural cavities
    
    • e.g. pleural effusion

Question 3

What determines how many x-rays get through the body?

Answer 3

e-density

• xrays interact with electrons
  
  • • electrons or close together, greater reaction
  • high atomic # things and high concentrations of e- e.g. metals (xrays cannot get through) are white
  • air is low e- density, tf black
  • fat, soft tissue (muscle, viscera) is grey
  • calcium of bone is white
  • contrast agents like iodine and barium can be used
• different e-densities side by side = silhouette sign
• same e-densities side by side cannot be differentiated

Question 4

What determines a ‘good’ CXR?

Answer 4

• if full inspiration:
  
  • 7 ribs anteriorly in the mid-clavicular line between the top of the chest and the diaphragm
  • 11 ribs posteriorly in the mid-clavicular line between the top of the chest and the hemidiaphragm
• silhouette in mid-clavicular line, not impeded by scapular border
• standing straight
  
  • line up spinous processes in the posterior midline to the trachea in the anterior midline
  • line up spinous processes with the sternal notch between the medial clavicles
• includes the costophrenic recess, where fluid would be

Question 5

What structures outline the mediastinum on CXR?

Answer 5

Question 6

How is heart size assessed on CXR?

Answer 6

maximum transverse diameter of the heart must be less than 50% of the maximum transverse internal diameter of the thoracic cavity

(must be erect, PA, full inspiration)

only in adults; does not work if hyperinflated (e.g. smokers, COPD, airway obstruction)

Question 7

How can lung fissures be seen in CXR?

Answer 7

In lateral view

Question 8

Pleural cavities on CXR

Answer 8

• are only seen if fluid/something is in then
• on R, need to look at lateral view
• fluid sinks, air rises
• fluid open to the atmosphere will be level; fluid not exposed to atomspheric pressure will have a meniscus due to negative intrapleural pressure
• pneumothorax is gas/air in the pleural space that separates the lung from the chset wall
• pleural effusion is an accumulation of fluid in the intrapleural space